Method for purifying the exhaust gases of an internal combustion engine having a catalytic converter

ABSTRACT

The present invention relates to a method for purifying the exhaust gases of an internal combustion engine having a catalytic converter which comprises oxygen storage components. The invention is concerned particularly with the restoration of the optimum filling degree of the oxygen storage components for regulated stoichiometric operation after the engine has been operated under lean conditions for a relatively short or relatively long period of time.

The present invention relates to a method for purifying the exhaust gases of an internal combustion engine having a catalytic converter which comprises oxygen storage components. The invention is concerned particularly with the restoration of the optimum filling degree of the oxygen storage components for regulated (lambda controlled) stoichiometric operation of the engine after it has been operated under lean conditions.

To purify the exhaust gases of such engines, use is made of so-called three-way catalytic converters which simultaneously remove carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx) from the exhaust gas.

The air ratio lambda (λ) is often used to describe the composition of the air/fuel mixture supplied to the engine. Said air ratio is the air/fuel ratio normalized in relation to stoichiometric conditions. The air/fuel ratio describes how many kilograms of air are supplied to the internal combustion engine per kilogram of fuel. The air/fuel ratio for a stoichiometric combustion is 14.7 for common engine fuels. At this point, the air ratio lambda is 1. Air/fuel ratios below 14.7, or air ratios below 1, are referred to as rich and air/fuel ratios above 14.7, or air ratios above 1, are referred to as lean.

If no storage effects for certain components of the exhaust gas occur in the internal combustion engine, then the air ratio of the exhaust gas corresponds to the air ratio of the air/fuel mixture supplied to the engine. To obtain a high degree of conversion of all three pollutants, the air ratio lambda must be set in a very narrow range around λ=1 (stoichiometric condition). The interval around λ=1 in which all three pollutants are at least 80% converted is often referred to as the lambda window.

To compensate fluctuations of the oxygen content in the exhaust gas, three-way catalytic converters comprise oxygen storage components (OSC) which store oxygen under lean exhaust-gas conditions (λ>1) and discharge oxygen under rich exhaust-gas conditions (λ<1) and thereby set the stoichiometry of the exhaust gas to λ=1. Compounds which permit a change in their oxidation state are suitable as oxygen storage components in a catalytic converter. Use is preferably made of cerium oxide, which may be present both as Ce₂O₃ and also CeO₂. To stabilize the cerium oxide, it is used for example as a mixed oxide with zirconium oxide.

Below, the storage capacity of the oxygen storage components is to be understood to mean the mass of oxygen which can be absorbed by the oxygen storage component per gram. Accordingly, the filling degree refers to the ratio of the actually stored mass of oxygen to the storage capacity. The storage capacity may be determined experimentally using various methods known to a person skilled in the art.

The aim of the regulation of the air ratio is to prevent a complete filling or a substantial emptying of the oxygen storage. In the event of complete filling of the oxygen store, a breakthrough of lean exhaust gas occurs and therefore nitrogen oxides are emitted. In the event of a substantial emptying, rich breakthroughs occur, that is to say carbon monoxide and hydrocarbons are emitted.

The signal of an oxygen probe (lambda probe) which is arranged upstream of the catalytic converter (pre-cat probe) in the flow direction of the exhaust gas is used for regulating the air ratio. By means of said probe, the air/fuel mixture supplied to the engine is regulated such that the exhaust gas is of stoichiometric composition before entering the catalytic converter. Within the context of this invention, said regulation is referred to as lambda regulation. An oxygen probe is usually incorporated in the drive train down-stream of the catalytic converter in addition to the pre-cat probe. The target stoichiometry of the lambda regulation can be re-adjusted by means of said post-cat probe. This is referred to as post-cat regulation. Post-cat regulation serves in particular for monitoring and adjusting the filling degree of the oxygen storage of the catalytic converter.

The probes generate an electrical voltage as a function of the oxygen content of the exhaust gas. For this purpose, use is conventionally made of two-point lambda probes, which are also referred to as step change lambda probes. Under lean exhaust gas conditions, said lambda probes have a voltage of approximately 0.2 V, which jumps from 0.2 V to over 0.7 V in a very narrow lambda interval at the transition to rich exhaust gas. Here, the postcat regulation is configured so as to yield a probe voltage of approximately 0.65 V. This point lies on the steepest branch of the probe characteristic curve and corresponds to an optimum filling degree of the oxygen storage of approximately 50%. In this way, upward or downward deviations from the stoichiometry of the exhaust gas can be easily detected and corrected.

A spark-ignition engine is operated predominantly with air/fuel mixtures of stoichiometric composition. However, if the engine is to no longer output power, the fuel supply is conventionally cut off. In the event of this so-called overrun fuel cutoff, only air is supplied to the engine, such that the exhaust-gas composition corresponds to the ambient air.

During the overrun fuel cutoff, the oxygen storage components of the catalytic converter are completely saturated, or filled, with oxygen. During the overrun fuel cutoff, post-cat regulation is not possible. Aside from the overrun fuel cutoff, a complete filling of the oxygen storage may also occur in other driving situations, for example on account of regulating errors of the lambda regulation.

After the end of an overrun fuel cutoff, regulated stoichiometric operation should be resumed as quickly as possible. For this purpose, however, it is firstly necessary for the filling degree of the oxygen storage to be returned to its optimum value of approximately 50%. For this reason, the engine is conventionally briefly operated with a rich air/fuel mixture after an overrun fuel cutoff. Said brief operation with a rich air/fuel mixture is also referred to as a rich pulse. Only when the filling degree of the oxygen storage has been returned to approximately 50% is regular post-cat regulation resumed. Alternatively, it is also known for the post-cat regulation to be reactivated directly after the end of the overrun fuel cutoff. Both methods have the disadvantage that it takes a relatively long time to set the optimum conditions for the lambda regulation. Undesired emissions may occur during said time period.

DE 10 2004 038 482 B3 is concerned with setting the filling degree of the oxygen storage after a transient operating state of the engine, such as for example an overrun fuel cutoff. In the event of the overrun fuel cutoff, the oxygen storage should be quickly emptied to an optimum value of approximately 50% of its filling degree. For this purpose, a rich air/fuel ratio λ<1 is set and then adjusted back toward 1 with optimum speed.

DE 10 2004 019 831 A1 prevents an undesired oxygen loading of the exhaust-gas catalytic converter during an overrun fuel cutoff phase by virtue of a catalytic converter mass flow with a defined, predetermined lambda value being supplied to the catalytic converter.

DE 10 2006 044 458 A1 is likewise concerned with the fuel injection after an overrun fuel cutoff. Here, during the first fuel injection after the ending of the overrun fuel cutoff, the fuel pulse width is set such that a fuel supply quantity is significantly increased in relation to an inlet air quantity, and the ignition time is set to a first retarded ignition time. During the second and subsequent fuel injections, a fuel pulse width is set which has a smaller increase width of the fuel, and the ignition time is set to a second retarded ignition time which is retarded to a lesser extent than the first retarded ignition time.

The inventors have observed that, in the known methods, the rich pulse after an overrun fuel cutoff leads to a temporary emission of carbon monoxide and hydrogen. Said emissions last for approximately 100 seconds and, at a maximum, have a concentration of 10 to 500 ppm carbon monoxide, as a result of which the post-cat regulation after the overrun fuel cutoff is disrupted and delayed.

It is therefore an object of the invention to specify a method by means of which the transition from the overrun fuel cutoff to regulated stoichiometric operation can be accelerated.

The object is achieved by means of the method defined in the main claim. Preferred embodiments are claimed in the subclaims.

The method relates to the purification of the exhaust gases of an internal combustion engine having a catalytic converter which comprises an oxygen storage composed of oxygen storage components, with the engine being equipped with an electronic engine controller and being operated with a regulated, stoichiometric air/fuel mixture over the greater part of the operating duration thereof, with temporary lean operating phases also occurring as a function of the driving situations.

The method is characterized in that, after a temporary lean operating phase of the engine with a lean air/fuel mixture, which is associated with a substantial filling of the oxygen store, and before the resumption of regulated engine operation, the filling degree of the oxygen storage is returned to an optimum level for stoichiometric operation by virtue of the engine being supplied with a rich pulse followed by a lean pulse, with the quantity of oxidative components supplied to the catalytic converter by means of the lean pulse being lower than would be required for fully compensating the quantity of rich exhaust-gas components supplied by means of the rich pulse.

The invention is based on the observation that, after an overrun fuel cutoff, the optimum filling degree of the oxygen storage for the stoichiometric regulation of the air/fuel ratio can be restored very quickly if a short rich pulse after the overrun fuel cutoff is followed by a short lean pulse. Here, the rich pulse and lean pulse are generated by means of corresponding control of the air/fuel ratio supplied to the engine. This preferably occurs by virtue of the pre-cat lambda probe predefining a corresponding chronological lambda profile. After the lambda profile has expired and the optimum filling degree of the oxygen storage has been reached, which can be identified by a post-cat signal voltage of approximately 0.6 to 0.7 volts, preferably 0.65 volts, the regular lambda regulation with pre-cat regulation and post-cat regulation is resumed.

The inventors have found that the oxidation (filling) or reduction (emptying) of the oxygen storage in the exhaust gas constitutes an equilibrium process.

An article by the inventors with the title “Is Oxygen Storage in Three Way Catalysts an Equilibrium Controlled Process?” from the journal “Applied Catalysis B: Environmental” was accepted for publication.

In steady-state operation, an equilibrium state of the oxygen storage is always set with the reducing and oxidizing components of the exhaust gas, that is to say in the equilibrium state, the reduction of the oxygen storage by means of carbon monoxide, hydrogen or hydrocarbons is compensated exactly by means of a corresponding oxidation with carbon dioxide and water.

An important consequence of said equilibrium behavior is that the maximum attainable degree of emptying of the store is dependent on the stoichiometry of the exhaust gas. For example, at lambda=0.95, the store is reduced (emptied) more completely than at lambda=0.99.

A further consequence of the equilibrium behavior is that a completely emptied oxygen storage is also partially oxidized again by moderately rich exhaust gas until a new equilibrium state is set with the moderately rich exhaust gas. Here, the oxygen store, by means of reaction with water or carbon dioxide, forms the components of carbon monoxide and hydrogen. Said situation arises if, according to the prior art, after an overrun fuel cutoff, the oxygen storage is emptied again only with a rich pulse. By means of said single rich pulse, the oxygen storage is significantly reduced (thoroughly emptied). If said thoroughly emptied oxygen storage is acted on with stoichiometric or slightly rich exhaust gas after the rich pulse, it generates carbon monoxide and hydrogen for a time period of 10 to several hundred seconds. Typical concentrations of said carbon monoxide and hydrogen release are approximately 10 ppm to 500 ppm. Said pollutant release may be reduced slightly if the rich pulse is not ended abruptly but rather is returned slowly to the stoichiometric value. However, this increases the time period between the end of the overrun fuel cutoff and the resumption of regulated operation, with the risk of further pollutant emissions.

In a more precise analysis of said processes, it is also necessary to take into consideration the distribution of oxidation and reduction of the oxygen storage along the catalytic converter. The rich pulse impinges firstly on the inlet end surface of the catalytic converter. Even if the rich pulse is dimensioned such that it can only partially empty the entire oxygen storage of the catalytic converter, thorough emptying of the oxygen storage nevertheless occurs in the front part of the catalytic converter and therefore, as a result of the pulse, a delayed release of carbon monoxide and hydrogen occurs. In said process, the rear part of the catalytic converter is only partially emptied. In the most favorable case, the carbon monoxide released from the front part of the catalytic converter and the hydrogen can empty the rear part of the catalytic converter to the desired extent. However, in this case, on account of the slowness of the carbon monoxide and hydrogen release, it takes 10 to 100 seconds until the oxygen storage has been completely emptied over the entire length of the catalytic converter and the stoichiometry of the exhaust gas downstream of the catalytic converter corresponds to the steady-state value.

The above-described carbon monoxide and hydrogen emissions in a vehicle operated according to the prior art have an adverse effect on the stability of the resuming post-cat regulation. For the post-cat regulation, use is made of a probe arranged downstream of the catalytic converter. As a result of the carbon monoxide and hydrogen emissions generated in the catalytic converter, the post-cat regulation is misled by the overall rich exhaust gas. The post-cat regulation attempts to compensate the rich offset by setting the air/fuel mixture supplied to the engine to be leaner. As a result of said leaning, the oxygen storage is filled with oxygen again, contrary to the actual purpose of the regulation. In the filled state, a breakthrough of nitrogen oxide occurs in the event of the slightest lean deviation of the exhaust gas. A consequence of the phenomenon mentioned is that it often proves to be difficult to switch to correct operation of the post-cat regulation after an overrun fuel cutoff. One solution to said problem is to deactivate the post-cat regulation for a certain period of time after an overrun fuel cutoff. Said solution is however not optimal because the catalytic converter is then operated in an unregulated fashion over a relatively long period of time.

The mentioned carbon monoxide and hydrogen emissions following a substantial reduction of the oxygen storage have an adverse effect not only after an overrun fuel cutoff. In normal operation, too, brief regulating errors can occur in particular in dynamic operating phases, which regulating errors lead to a complete filling of the oxygen store. When the store is substantially filled and at the same time the stoichiometry of the exhaust gas briefly deviates into the lean range, a lean breakthrough occurs which is registered by the post-cat probe. As described in the introduction, the signal of the post-cat probe is used to re-adjust the target stoichiometry of the lambda regulation. In this case, the post-cat regulation results in the air/fuel mixture supplied to the engine being enriched again. Said enrichment has a similar effect as the rich pulse after an overrun fuel cutoff: the oxygen storage is firstly emptied very thoroughly. Said thorough emptying results in the above-described carbon monoxide and hydrogen emissions. The post-cat regulation reacts to this with a leaning of the air/fuel mixture supplied to the engine, which can lead to a renewed lean breakthrough with a fall in the post-cat probe voltage. The fall in the post-cat probe voltage starts the described process of carbon monoxide and hydrogen emissions, leaning and lean breakthrough from the beginning. A periodic oscillation of the exhaust-gas stoichiometry with periodic lean breakthroughs and corresponding nitrogen oxide emissions thus occurs. Said oscillating behavior is well known to a control engineer. To prevent oscillations, the response time of the regulation must be increased by adjusting the regulating parameters. Said solution is of course not optimal because, as a result of the reduced speed of the regulation, the lambda deviations which inevitably occur in driving operation can be compensated only with an unnecessarily lengthened response time.

According to the invention, the stated problems of the conventional method are reduced or even completely eliminated in that, after the end of the lean operating phase, the filling degree of the oxygen storage is returned to the optimum level for the subsequent post-cat regulation by means of at least one rich pulse and one lean pulse.

Here, it is preferable for the quantity of rich exhaust-gas components to be greater than that required for setting the optimum filling degree for stoichiometric operation but smaller than the quantity of rich exhaust-gas components which would be required for completely emptying the storage capacity of the oxygen store.

According to the invention, therefore, firstly a rich pulse is used which is capable of emptying the catalytic converter over the entire length thereof. Here, the front part of the store is thoroughly emptied. Said thorough emptying in the front part is ended by means of a relatively small lean pulse. To also fill the rear part of the previously thoroughly emptied zone, the lean pulse will inevitably fill a small zone at the inlet of the catalytic converter beyond the optimum filling degree again. This may be compensated by means of a further rich pulse which is selected such that the quantity of rich components provided by it is smaller than that required for completely compensating the preceding lean pulse.

The reducing agent quantity in the first rich pulse must be greater than the equivalent quantity of oxygen which must be extracted from the catalytic converter at the transition from the fully oxidized state into the stoichiometric operating state. The catalytic converter is thus firstly thoroughly emptied. The reducing agent quantity in the first rich pulse is however preferably selected to be smaller than the equivalent oxygen quantity which can be extracted from the catalytic converter by means of a steady-state rich operating mode.

The pulse sequence is preferably configured, as a function of the operating state of the engine and the aging state of the catalytic converter, such that, after the end of the pulse sequence, the store loading distribution corresponds to the distribution which would also be set during regulated operation of the catalytic converter at said operating point. An optimum pulse sequence may be identified in that, after the end of the pulse sequence, the voltage of the post-cat probe assumes, in a stable manner, the setpoint value of the post-cat regulation. The amplitude and/or the duration of the rich pulse and lean pulse are available as influential variables for said optimization. The amplitude and/or duration may be optimized as a function of the temperature and spatial velocity of the exhaust gas and/or an aging state of the catalytic converter.

Should the sequence of a rich pulse and lean pulse be insufficient for completely returning the filling degree to the optimum value, the engine may be supplied with further rich and lean pulses after the first rich pulse and lean pulse, with the quantity of rich components supplied with the respective rich pulse being greater than that which can be compensated with the oxidative components of the subsequent lean pulse. The optimum number of successive lean/rich pulses may be determined in preliminary tests as a function of the operating conditions after an overrun fuel cutoff.

The method is preferably used for the exhaust-gas purification of stoichiometrically operated internal combustion engines in which overrun fuel cutoffs occur if engine power is no longer required. In this case, the overrun fuel cutoffs form the temporary lean operating phases. Temporary lean operating phases may however also be caused by undesired regulation fluctuations of the stoichiometric operation.

A further field of use of the invention is the exhaust-gas purification of a lean-operated internal combustion engine which is operated partially stoichiometrically and partially lean. For low power demands in city traffic, the engine is operated lean in order to save fuel. If higher levels of power are demanded, the engine must be switched to stoichiometric operation. It is thus the case here that the oxygen storage in the catalytic converter is completely filled in the lean operating mode, in exactly the same way as in the event of an overrun fuel cutoff. The switch to stoichiometric operation leads to the same problems as those encountered after an overrun fuel cutoff.

Undesired temporary lean operating phases as a result of a regulating error are preferably detected in that the post-cat probe indicates lean exhaust gas. For this purpose, use may be made of a step change probe. If the signal voltage thereof falls below a predefined threshold value, then a temporary lean operating phase according to this invention is present. The threshold value may be selected as a function of the temperature and the spatial velocity of the exhaust gas, as a function of the exhaust-gas stoichiometry and as a function of the aging state of the catalytic converter. Said threshold values are preferably stored in a table of the engine controller.

The oxygen storage components of the exhaust-gas purification catalytic converter continuously lose storage capacity as a result of thermal aging. The method makes it possible to determine the storage capacity still remaining. The output signal of the oxygen probe arranged downstream of the catalytic converter in the exhaust section may be used for this purpose. If the signal voltage lies below the expected voltage after the step from the temporary lean operating phase into regulated stoichiometric operation, then the remaining oxygen storage capacity of the catalytic converter is lower than assumed. In this way, it is thus possible to determine the remaining oxygen storage capacity from the signal voltage in the stoichiometric operating mode after an overrun fuel cutoff. If the remaining oxygen storage capacity falls below a predefined value, then a corresponding warning signal can be activated.

The determination of the remaining oxygen storage capacity makes it possible to adapt the quantity of rich and lean components supplied to the catalytic converter by means of the rich and lean pulses to the remaining oxygen storage capacity, and to thereby optimize the transition from the overrun fuel cutoff to regulated stoichiometric operation. This preferably takes place by virtue of the amplitudes of the rich and lean pulses being reduced by a factor corresponding to the remaining oxygen storage capacity. The factor may be stored in a table of the engine controller as a function of the remaining oxygen storage capacity.

It is advantageous to store a mean value for the oxygen storage capacity in the engine controller, from which mean value the oxygen storage capacity for the different operating points of the engine can be calculated by means of a corrective factor.

The invention will be explained in more detail on the basis of the following figures, in which

FIG. 1: shows the release of carbon monoxide/hydrogen in the stoichiometric operating mode following a rich pulse.

FIG. 2: shows a conventional lambda profile after an overrun fuel cutoff and the resulting profile of the voltage of the lambda probe down-stream of the catalytic converter for two different rich pulses after an overrun fuel cutoff

FIG. 3: shows a lambda profile according to the invention after an overrun fuel cutoff and the resulting profile of the voltage of the lambda probe downstream of the catalytic converter

FIG. 1 shows the emissions of carbon monoxide and hydrogen after an overrun fuel cutoff and a return to stoichiometric operation by means of a single rich pulse. For said measurements, a conventional three-way catalytic converter was tested in a model gas system.

The upper diagram shows the profile of the air ratio lambda as a function of the time (lambda profile). During the first 10 seconds, an overrun fuel cut-off with a lambda value of 1.1 was simulated. After the end of the overrun fuel cutoff, the oxygen storage of the tested three-way catalytic converter was emptied, by means of a single rich pulse, to the filling degree for the stoichiometric operation with lambda=1. The two lower diagrams show in each case the measured profile of the hydrogen and carbon monoxide concentrations downstream of the catalytic converter. After a time delay after the rich pulse, hydrogen and carbon monoxide are released by the catalytic converter. The emissions of said two pollutants last for a period of longer than 40 seconds.

FIG. 2 shows the result of simulation calculations for the case of a conventional lambda profile after an overrun fuel cutoff with complete filling of the oxygen store. The calculations were carried out for two rich pulses of different length with a lambda value of 0.9. The lambda profiles upstream of the catalytic converter are shown in the upper diagram. The lower diagram shows the calculated signal voltages of the post-cat probe.

The signal voltage of the post-cat probe starts at approximately 0.1 V and indicates a very lean exhaust gas (lean operating phase) with a high oxygen component. The oxygen storage has virtually a 100% filling degree. To empty the oxygen store, the exhaust gas is briefly enriched upstream of the catalytic converter.

For a duration of the rich pulse of only 1.0 seconds (dashed curves), it takes approximately 17 seconds for the signal voltage of the post-cat probe to rise to 0.65 V. For a duration of the rich pulse of 1.4 seconds, a signal voltage of 0.65 V is reached already after only approximately 3.5 seconds. In both cases, however, the post-cat probe registers a further shift of the stoichiometry of the exhaust gas toward rich values. After 40 seconds, the probe voltage remains at approximately 0.75 V. This significant rich shift is caused by the above-described emissions of carbon monoxide and hydrogen.

FIG. 3 shows the result of simulation calculations for the case of a lambda profile according to the invention. In this example, to empty the oxygen store, the exhaust gas upstream of the catalytic converter has two pairs of rich and lean pulses with an overall duration of approximately 20 seconds. The diagram with the signal voltage of the post-cat probe reaches the desired 0.65 V, and remains at this voltage level, already after approximately 4 seconds. The oxygen storage has thus reached an optimum filling level, averaged over its entire length, already after said short time with only one rich/lean pulse pair. Nevertheless, on account of the above-described axial distribution of the filling level, a further rich/lean pulse pair is required to optimally set the filling degree over the entire length of the catalytic converter. The post-cat regulation remains deactivated after the end of the preceding lean operating phase at the time zero until the end of the final rich/lean pulse pair at approximately 20 seconds. Only thereafter is the post-cat regulation resumed. 

1. A method for purifying the exhaust gases of an internal combustion engine having a catalytic converter which comprises an oxygen storage composed of oxygen storage components, with the engine being equipped with an electronic engine controller and being operated with a regulated, stoichiometric air/fuel mixture over the greater part of the operating duration thereof, with temporary lean operating phases also occurring as a function of the driving situations, wherein, after a temporary lean operating phase of the engine, which is associated with a substantial filling of the oxygen store, and before the resumption of regulated engine operation, the filling degree of the oxygen storage is returned to an optimum level for stoichiometric operation by virtue of the engine being supplied with a rich pulse followed by a lean pulse, with the quantity of oxidative components supplied to the catalytic converter by means of the lean pulse being lower than would be required for fully compensating the quantity of rich exhaust-gas components supplied by means of the rich pulse.
 2. The method as claimed in claim 1, wherein the quantity of rich exhaust-gas components supplied by means of the rich pulse is greater than that required for setting the optimum filling degree for stoichiometric operation but is smaller than the quantity of rich exhaust-gas components which would be required for completely emptying the storage capacity of the oxygen store.
 3. The method as claimed in claim 2, wherein, after the first rich pulse and lean pulse, the engine is supplied with further rich and lean pulses, with the quantity of rich components supplied by means of the respective rich pulse being greater than could be compensated by means of the oxidative components of the subsequent lean pulse.
 4. The method as claimed in claim 1, wherein the rich pulse and lean pulse have an amplitude and a duration and the amplitude and/or duration are adapted as a function of the temperature and spatial velocity of the exhaust gas and/or an aging state of the catalytic converter.
 5. The method as claimed in claim 4, wherein the amplitudes of the rich pulse and lean pulse are reduced by a factor corresponding to the aging state of the catalytic converter.
 6. The method as claimed in claim 1, wherein the temporary lean operating phase is an overrun fuel cutoff.
 7. The method as claimed in claim 1, wherein the temporary lean operating phase is a lean operating phase of an internal combustion engine operated both stoichiometrically and also lean as a function of the driving situation.
 8. The method as claimed in claim 1, wherein the temporary lean operating phase is caused by regulation fluctuations of the stoichiometric operation.
 9. The method as claimed in claim 8, wherein the temporary lean operating phase is detected in that an oxygen probe arranged downstream of the catalytic converter indicates lean exhaust gas when the signal voltage of said oxygen probe falls below a threshold value.
 10. The method as claimed in claim 9, wherein the threshold value is selected as a function of the temperature and the spatial velocity of the exhaust gas, as a function of the exhaust-gas stoichiometry and as a function of the aging state of the catalytic converter.
 11. The method as claimed in claim 1, wherein an oxygen probe is arranged in the exhaust section downstream of the catalytic converter and the signal voltage which said oxygen probe actually reaches after the step from the temporary lean operating phase into regulated stoichiometric operation is used to determine therefrom a remaining oxygen storage capacity of the oxygen storage.
 12. The method as claimed in claim 11, wherein a signal is activated if the remaining oxygen storage capacity has fallen below a predefined value.
 13. The method as claimed in claim 12, wherein the quantity of rich and lean components supplied to the catalytic converter by means of the rich and lean pulses is adapted to the remaining oxygen storage capacity.
 14. The method as claimed in claim 2, wherein the rich pulse and lean pulse have an amplitude and a duration and the amplitude and/or duration are adapted as a function of the temperature and spatial velocity of the exhaust gas and/or an aging state of the catalytic converter.
 15. The method as claimed in claim 14, wherein the amplitudes of the rich pulse and lean pulse are reduced by a factor corresponding to the aging state of the catalytic converter. 